GABAergic interneurons perform crucial roles in cortical development and function. These roles are executed by diverse interneuron subtypes, and abnormal function of particular interneurons has been implicated in a variety of neuropsychiatric diseases. However, little is known about the molecular mechanisms that underlie interneuron fate determination. Many cortical interneurons originate in the ventral (subcortical) telencephalon, where recent studies have begun to further define the origins of distinct subgroups of cortical interneurons. The realization that distinct interneuron subgroups have distinct origins has begun to inform efforts to understand the molecular regulation of interneuron specification. For example, in the medial ganglionic eminence (MGE) of the subcortical telencephalon, the signaling molecule Sonic Hedgehog (Shh) maintains the expression of Nkx2.1, a transcription factor whose expression during neurogenesis is required for the specification of important subgroups of cortical interneurons (Xu &Anderson, 2004, 2005). The goal of the experiments described below is to determine the mechanisms behind the specification of distinct interneuron subgroups within the MGE. Our overriding hypothesis is that much of the fate diversity of MGE- derived interneurons is based on their distinct origins within the MGE, and by the molecular differences encountered by progenitors within these origins. Aim 1. Origins of Parvalbumin or Somatostatin-expressing interneuron subgroups in the MGE. Expression of the neuropeptide somatostatin (SST) or the calcium binding protein parvalbumin (PV) define two non-overlapping subgroups of cortical interneurons, both of which require Nkx2.1 for their specification. Preliminary data from transplantation studies suggest that there is a strong bias for the SST+ subgroup to originate from the dorsal MGE, and for the PV+ subgroup to originate from the ventral MGE. This aim will further examine the spatio-temporal origins of these interneuron subgroups in the MGE. Aim 2. Specification of Parvalbumin or Somatostatin-expressing interneuron subgroups in the MGE. Nkx6.2, like the Shh signaling effector Gli1, is selectively expressed in the dorsal-most region of the MGE. Both of these genes are downregulated when Shh signaling is inactivated in the neuroepithelium by NestinCre:SmoFl/Fl (Xu &Anderson, 2005). The role of Nkx6.2 in interneuron specification will be tested by loss of function studies-- analysis of Nkx6.2 nulls, and gain of function studies--analysis of the interneuron fate effect of transient misexpression of Nkx6.2 in the ventral MGE of wild type slices, and of the ability of Nkx6.2 to rescue interneuron fate in Nkx2.1-/- slices. Future experiments will test the functions of other genes with enriched expressed in the dorsal or ventral MGE that have been identified through an array based screen.